For many years, adult haemopoietic stem cells (HSCs) have been considered 'plastic' in their proliferative and differentiation capacities. Recently, evidence that supports newer concepts of adult stem cell plasticity has been reported. In particular, stem cells from haemopoietic tissues seem to have 'extraordinary' abilities to generate or switch between haemopoietic and nonhaemopoietic lineages, exhibiting an unexpected degree of developmental or differentiation potential. The mechanisms by which cell fate reprogramming occurs are still poorly understood. Nevertheless, an increasing number of studies is challenging one of the main dogmas in biology, namely that mammalian cell differentiation follows established programmes in a hierarchical fashion, and once committed to a particular somatic cell lineage, cells do not change into another somatic lineage. The 'nonhierarchical', 'reversible' phenotype of stem cells in haemopoietic tissues, if it exists, would be an advantage that could be exploited in regenerative medicine. Here, we review the recent advances in HSC biology and discuss the general concepts of adult stem cell plasticity with respect to these cells and how these might be exploited clinically.